Regulations in force in the great majority of countries impose three conditions for the operation of safety valves:
1. The safety valve must begin to open as soon as the pressure of the hydraulic circuit exceeds the rated level; in the case of a pneumo-hydraulic device, this is equivalent to the test value of the hydraulic accumulators.
2. The safety valve must allow the fluid to be removed towards the low-pressure tank following an anomaly constituted by an untimely rise in pressure, e.g. when the motor-driven pump does not stop because of faulty operation of its power supply switch. The pressure in the hydraulic circuit must not exceed the maximum permissible value by 10%.
3. Accidental damage to a component of the safety valve itself should not cause the main valve to be blocked on its seat; consequently the pressure to be controlled must have effect in the direction of the opening of the valve. This excludes the use of a pilot valve.
Experience shows that a valve which fulfills the last condition, i.e. a valve pressed against a seat by a calibrated spring and which is subjected to pressure has, as far as concerns its closing threshold, a response which varies according to the rate of discharge through it; this rate of discharge can range from drop-by-drop in the case of thermal expansion of the fluid, to pump discharge in the case of accidental operation of the motor which drives the pump. The higher the discharge rate, the greater the difference between the opening pressure and the complete closing pressure.
Therefore, in conventional hydraulic assemblies, it is necessary to take this phenomenon into account when fixing the starting threshold of the motor-driven pump. The threshold must be below the lowest closing threshold of the valve; in the contrary case, fluid would be pumped: the valve would never be completely sealed and the motor-driven pump would start up at regular intervals. However, to fulfill such a condition, requires the pump starting threshold and consequently the minimum operation pressure threshold of the installation, to be adjusted well above the maximum permissible rated pressure. This finally results in improper use of the installation from the point of view of energy efficiency.
The present invention aims to provide a device which ensures proper operation of the motor-driven pump while maintaining an operation pressure threshold very close to the maximum permissible rated pressure.